Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-04-2017, 09:06 PM   #1
Senior Member




Join Date: Jun 2015
Location: Melbourne, Australia
Photos: Albums
Posts: 242
What does the lens transmit to the body re: shutter speed?

Even with in body stabilization, it makes sense that the shutter speed should decrease (become faster) the longer the focal length. It seems this is what the camera is doing when I put for example the FA* 300 on my camera. However, when I put an older all-manual lens like the Tamron 500mm mirror on my camera and tell the camera "500mm" when it boots, the shutter speed selected is remarkably slow: 1/125, even though the camera could choose a higher ISO and thus decrease the shutter speed. But with the FA*, the shutter speed goes to much smaller numbers, like 1/300 or 1/400. So is the newer lens transmitting more info to the camera to tell it to do that?

Example: for the Tamron 500, the camera will select 1/125 and keep the ISO at 800, whereas for the FA* it will select 1/400 put the ISO all the way to 3200. What's the deal?

03-04-2017, 09:19 PM   #2
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 46,483
The camera probably does this because it doesn't want to rely on (potentially inaccurate) user input. Just my guess.

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

03-04-2017, 09:40 PM   #3
Senior Member
aremmes's Avatar

Join Date: Oct 2013
Location: Philadelphia
Photos: Albums
Posts: 215
Lenses in the FA and newer series transmit MTF data, informing the camera about characteristics related to resolution and contrast among other things. The camera can then adjust exposure parameters to maximize resolution given the available light. Without the MTF data, the camera defaults to exposing to middle gray in the selected metering mode.
03-04-2017, 10:02 PM   #4
Pentaxian
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 29,629
The information is more limited than one might think and varies according to lens and body generation. At base:
  • lens ID code
  • Current maximum aperture (zooms)
  • Current focal length (variable aperture zooms)
  • Focus ring position
In addition, more recent models add:
  • Focus motor status
  • Aperture motor status
The above is all read through the data pin. In addition, both power and control messaging is provided through the "power zoom" contacts on the inner part of the mount to operate in-lens focus and aperture actuator motors.

Other lens-related information (MTF data, lens correction data, lens capability matrices, and operations codes) are flashed on to the body's firmware as lookup tables. That is why new lenses often require firmware updates for full compatibility.


Steve


Last edited by stevebrot; 03-05-2017 at 01:07 AM.
03-05-2017, 01:05 AM   #5
Pentaxian
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 29,629
QuoteOriginally posted by automorphism Quote
Example: for the Tamron 500, the camera will select 1/125 and keep the ISO at 800, whereas for the FA* it will select 1/400 put the ISO all the way to 3200. What's the deal?
I don't believe that the body tailors the exposure program based on focal length. At least that has not been my experience with my K-3. A few questions:
  • What Adaptall-2 mount are you using with the Tamron? PK or PKA?
  • What camera body and exposure mode are you using?
  • What program line are you using (exposure menu 1, Program Line)
A few comments:
  • Since your Tamron is fixed-aperture, it is unclear how the body will treat your lens when paired with the PKA adapter. Fixed aperture lenses are not defined for the contact pattern scheme nor is an f/8 maximum aperture supported by the PKA adapter.
  • Auto-ISO and the program line are semi-independent. I used to be up on the algorithm, but don't remember all the details except that under some circumstances it is fairly difficult to get the ISO or program line to ramp as one might expect. That is one reason why I like to complain that auto-ISO is evil except perhaps in TAv mode.


Steve

Last edited by stevebrot; 03-05-2017 at 08:45 AM.
03-05-2017, 01:47 AM - 1 Like   #6
Site Supporter
Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 1,884
Using a 500mm lens, in the old days would have meant you should select at least 1/500 shutter speed. With shake reduction you have more leeway, but I would still suggest 1/500 as your lowest. The camera has many modes on the dial such as Manual/Tv/TAV where you select the shutter speed. You should use those modes.
03-06-2017, 09:38 PM   #7
Senior Member




Join Date: Jun 2015
Location: Melbourne, Australia
Photos: Albums
Posts: 242
Original Poster
Thanks for the reply guys.

QuoteOriginally posted by stevebrot Quote
I don't believe that the body tailors the exposure program based on focal length. At least that has not been my experience with my K-3. A few questions:
  • What Adaptall-2 mount are you using with the Tamron? PK or PKA?
  • What camera body and exposure mode are you using?
  • What program line are you using (exposure menu 1, Program Line)
I am just using the PK adapter on a K-50, center-weighted, with the dial in Av mode (obviously, with the Tamron and Pk adapter I can't actually adjust the aperture). I use the same mode for the FA* lens.

I tested it some more, and here are more precise results:

With a Pentax-A 70-210, the camera will not increase the ISO unless the shutter speed would drop below 1/125 (this is the same with the Tamron 500mm). Whereas with the FA*300, it will increase the ISO whenever the shutter speed would otherwise drop below 1/400.

I always thought that the input focal length screen was to help the IBIS system out, so I figured that if I input a focal length it should increase the shutter speed for longer ones to compensate for the shake. But for some reason it keeps it at 1/125 for the Tamron, which seems a little slow for such a lens as pschlute pointed out.
03-06-2017, 11:13 PM   #8
Pentaxian
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 29,629
QuoteOriginally posted by automorphism Quote
I always thought that the input focal length screen was to help the IBIS system out
Focal length is an operating parameter for the IBIS system, but is not used by the exposure automation.

QuoteOriginally posted by automorphism Quote
But for some reason it keeps it at 1/125 for the Tamron, which seems a little slow for such a lens as pschlute pointed out.
The system is biased to keep ISO low in the interest of noise reduction though with a basement of 1/125s shutter speed if the auto-ISO low end is set to 100. With a slow fixed aperture lens the shutter speed pegs at 1/125s and the ISO shifts instead until its upper limit is reached.* I know...it is sort of backwards. A solution is to turn off auto-ISO and set the ISO at a level appropriate to the ambient lighting at f/8. Alternatively, if you want an automated 1/500s or higher with your Tamron, I suggest ISO 800 as the lower limit in the auto-ISO range.


Steve

* I actually tested this out using a manual aperture lens to emulate your Tamron. With auto-ISO on and f/8 maximum fixed aperture in Av mode and very bright light, the shutter speed stayed at 1/125 as the light intensity increased until the bottom value for auto-ISO was reached and only then would it ramp up.


Last edited by stevebrot; 03-06-2017 at 11:46 PM.
03-06-2017, 11:50 PM   #9
Senior Member




Join Date: Jun 2015
Location: Melbourne, Australia
Photos: Albums
Posts: 242
Original Poster
QuoteOriginally posted by stevebrot Quote
The system is biased to keep ISO low in the interest of noise reduction though with a basement of 1/125s shutter speed if the auto-ISO low end is set to 100. With a slow fixed aperture lens the shutter speed pegs at 1/125s and the ISO shifts instead until its upper limit is reached.* I know...it is sort of backwards. A solution is to turn off auto-ISO and set the ISO at a level appropriate to the ambient lighting at f/8. Alternatively, if you want an automated 1/500s or higher with your Tamron, I suggest ISO 800 as the lower limit in the auto-ISO range.
Yeah, lower limiting the ISO is a good idea. I'll definitely test that out, though the Tamron is already so fickle it's mostly just for fun.
03-07-2017, 05:06 AM   #10
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,447
QuoteOriginally posted by automorphism Quote
Even with in body stabilization, it makes sense that the shutter speed should decrease (become faster) the longer the focal length. It seems this is what the camera is doing when I put for example the FA* 300 on my camera. However, when I put an older all-manual lens like the Tamron 500mm mirror on my camera and tell the camera "500mm" when it boots, the shutter speed selected is remarkably slow: 1/125, even though the camera could choose a higher ISO and thus decrease the shutter speed. But with the FA*, the shutter speed goes to much smaller numbers, like 1/300 or 1/400. So is the newer lens transmitting more info to the camera to tell it to do that?

Example: for the Tamron 500, the camera will select 1/125 and keep the ISO at 800, whereas for the FA* it will select 1/400 put the ISO all the way to 3200. What's the deal?
My experience through the evolution of the Pentax DSLRs is that the use of focal length is explicitly for shake reduction, I too find this a bit odd, but even with KA lenses the camera does not consider the input focal length into the exposure, although it easily could do so, to give native exposure that is within the hand hold limits, but it seems to ignore this data.

There are a lot of funny things in the way exposure is programmed, such a pushing ISO up as opposed to using flash when available. The flash programming is done for fill flash, using natural light for the main exposure.

It is why o shoot a lot of manual, or, if out hunting with a 400mm lens, I use Tav mode and force shutter and aperture. I get what I want and live with grain., because the other solution is a blurry, but fine grained shot.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
body, camera, decrease, fa*, iso, k-mount, lens, pentax lens, shutter, slr lens, tamron
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
What is the single shutter speed available on an ESII without batteries? goatsNdonkey Pentax Film SLR Discussion 20 02-15-2017 10:45 PM
When your camera adjusts its shutter speed to the selected focus length? Prakticant Pentax K-3 6 03-14-2015 11:17 AM
What happened to the body cap lens? lenshoarder Pentax Q 5 07-08-2013 05:22 PM
The pins on the body, what does each do? Nass Pentax DSLR Discussion 2 09-16-2011 05:25 AM
What's the slowest Shutter Speed you use for handheld shots? mojoe_24 Troubleshooting and Beginner Help 32 02-11-2011 11:43 AM



All times are GMT -7. The time now is 03:11 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top